It is forbidden to do melacha on Yom Tov for Shabbat. Therefore, anyone who will prepare for Shabbat on Yom Tov by cooking, baking or by doing anything else that may not be done on Shabbat itself—must make (or be included in) an eruv tavshilin. While guests may rely on their host’s eruv; those who sleep at home may need to make one of their own, depending on the circumstances, and should ask a Rav.
On erev Yom Tov choose two cooked foods that will be eaten on Shabbat to serve as the eruv. Customarily we use a challah or matza (a baked food) and a significant cooked food, such as a piece of meat or fish. One then recites the nusach as printed in the siddur. The eruv must be in existence when preparing for Shabbat, and we eat it on the Shabbat following the Yom Tov. The custom is to eat it during Shabbat’s final meal, and to also use the eruv challah or matza as the second of the lechem mishneh for each of the Shabbat meals. The eruv only permits food related preparations for Shabbat on the second day of Yom Tov, namely erev Shabbat. All foods cooked for Shabbat on Yom Tov must be ready well before the onset of Shabbat (before candle lighting time at the very latest).
